The present invention relates to improved gravity displays and more particularly, to improved methods for isostatically correcting gravity data to generate more accurate gravity grids, maps or other displays.
Displays of the earth's gravity field in the form of various grids or maps have long been considered useful in determining subsurface geology and structure. Such displays have been used in searching for all types of minerals, including oil and gas accumulations. The instruments and methods for acquiring original gravity data are well known. It is also well known that numerous corrections must be made to the raw gravity measurements in order to generate displays which are actually useful for the geological studies and exploration purposes.
Raw gravity measurements are normally corrected for variations of gravity with latitude, for variations of gravity with elevation, also known as the free-air effect, and for variations due to attraction of near surface material, also known as the Bouguer effect. Commercially available gravity data sets usually include these corrections. Typically such commercial data sets include four versions of the data: the raw gravity measurements; the raw data corrected for latitude; the raw data corrected for latitude and free-air effect; and the raw data corrected for latitude, free-air effect and Bouguer effect. See, for example, the text entitled "Gravity and Magnetics in Oil Prospecting" by L. L. Nettleton, copyright 1976 by McGraw Hill, Inc. As also discussed in this text, further refinements to gravity displays include corrections for isostasy, which allow the preparation of isostatically corrected gravity displays. Generally stated, isostasy relates to variations in gravity readings caused by a deficiency in density under areas of high topography.
The isostatic correction is quite important for oil prospecting. Generally stated, surface features of high elevations, such as mountains, exhibit low gravity readings. Unfortunately, subsurface basins which indicate the possible presence of hydrocarbons, also produce low gravity anomalies. As a result, high elevations can easily mask the presence of the subsurface features which may be desirable from the point of view of the oil explorationist. The isostatic correction, if performed properly, can correct for the gravity lows produced by the surface features. If this is done accurately, any resulting low gravity features can indicate the presence of basins which are suitable for the accumulation of hydrocarbons. Thus, from the point of view of the oil explorationist, it is quite important that the isostatic corrections be made and that they made accurately.
Generally speaking, the basic methods for preparing isostatically corrected gravity maps are well known. See, for example, the publication "A New Isostatic Residual Gravity Map of the Conterminous United States With A Discussion of the Significance of Isostatic Residual Anomalies" by Simpson, et al., published in The Journal of Geophysical Research, Volume 91, No. B8, pages 8348-8372, Jul. 10, 1986. Simpson, et al. have also provided computer software useful in preparation of such maps which has been published in "Airyroot: A Fortran Program for Calculating the Gravitational Attraction of an Airy Isostatic Root Out to 166.7 Km" published by the United States Department of the Interior Geological Survey in Open-File Report 83-883, 1983. However, as discussed by Simpson, et al., the basic equation for the isostatic correction requires the use of three quantities: the crustal density, .rho.; the density contrast between lower crust and mantle, .DELTA..rho.; and the crustal thickness at sea level, T. In all prior isostatically corrected maps, these three quantities have been assigned assumed values generally based on prior publications. That is, they are not actually based on measured values. Since the accuracy of the isostatic corrections is clearly dependent upon the accuracy of these three values, the accuracy of the resultant isostatically corrected maps is subject to question.